A belt-type particleboard press has a press frame constituted as a plurality of frames traversed by upper and lower beams carrying respective upper and lower press platens that are normally heated. Endless upper and lower belts have stretches extending along vertically spaced from each other between the platens. These belts are driven synchronously to advance a mat workpiece through the press. A plurality of substantially identical hydraulic actuators are engaged between each frame and the upper beam and platen. The number of actuators per frame increases from the downstream end toward the upstream end. Thus the pressure exerted can be substantially greater at the upstream end than at the downstream end. These actuators are upright simple hydraulic rams and are all pressurized at the same pressure. As described in commonly owned U.S. Pat. No. 4,468,188, the distribution of the rams and of the frames establishes the desired pressing force and press strength in an upstream compression zone and a downstream calibration zone of the press.
Typically the mat is subjected to considerable heat and pressure in the upstream compression zone so as to activate the resin. The pressure is necessary for good heat transmission between the heated platens and the workpiece and to ensure good mixing of the binder with the fibers, chips, or other particles in the mat. The downstream region of the press operates at a lower pressure, so that the effect here is calculated to impart a predetermined finished thickness to the panel being pressed. The resistance that the workpiece offers to compression decreases as it is compressed and heated, in part since the binder plastifies, so that in the calibration zone the main effect achieved is stabilization of the thickness of the compressed panel as the normally thermosetting binder cures.
The problem with these arrangements is that the incoming workpieces vary within a fair range as regards density, thickness, and composition. Hence their resistance to compression and their compressability will vary correspondingly so that the constant force applied in the compression zone will overcompress some parts of some workpieces, thereby damaging fibers, and will not sufficiently compress other parts, thereby leaving voids.
Accordingly German patent document No. 2,343,427 describes an arrangement for dealing with this problem. The calibration zone is provided with strain gauges that measure the pressure with which the mat being calibrated resists compression. A controller compares these detected reaction pressures with desired values so that, for instance, when the reaction pressure drops to indicate the board is overly compressed, it reduces pressure upstream in the actuators bearing on the workpiece in the compression zone. Such a complicated arrangement operates adequately with slowly varying workpieces, but the feedback nature of its operation creates a response time too long to compensate out localized irregularities, and in fact can damage the workpiece in response to detection of such a localized problem.
In jointly filed and copending patent application Ser. No. 719,759, filed Apr. 4, 1985, spacers are provided for establishing the position in which the movable platen is held in the downstream zone. In addition the movable platen of this system has a compression portion that is at least limitedly vertically displaceable relative to the downstream calibration portion. The calibration portion is urged into a calibration position spaced a predetermined vertical distance from the other platen and the compression portion is urged with a predetermined generally constant force toward the other platen while permitting deflection of the compression portion of the one platen away from the other platen against this constant force and without substantial change of same.